Swivel chair base



FIG. 1 is the perspective view of a swivel chair base showing my new design;

FIG. 2 is the front elevation view thereof;

FIG. 3 is the rear elevation view thereof;

FIG. 4 is the left side view thereof;

FIG. 5 is the right side view thereof;

FIG. 6 is the top plan view thereof; and,

FIG. 7 is the bottom plan view thereof. 

The ornamental design for a swivel chair base, as shown and described. 